For reasons of security, increased information density, or aesthetics, a mark that is invisible to the eye and can be applied anywhere on the surface of a document or other article without masking or obscuring in any way the underlying background would be highly desirable. Of course, conventional visible mark scanners that rely on variations in light intensity reflected back from the surface of a substrate are unable to detect such an invisible mark.
U.S. Pat. No. 4,540,595 discloses methods for marking substrates with inks containing a fluorescent component that emits light in or close to the infrared region, the fluorescent emission being employed for signal processing in place of conventional reflected light. Use of compounds that fluoresce in the near infrared (NIR) region, i.e., in the range of about 700 nm to 1100 nm, are noted to be particularly attractive since there is little background interference compared to the UV region where many common compounds absorb and fluoresce, which makes possible improved signal to noise ratios. However, the compounds that are specifically disclosed in the reference lose their fluorescent capability too quickly to be practically useful.
Ideally, suitable fluorescent components would display Stokes shifts, defined as the difference between peak absorbance and peak fluorescence, that are large, preferably in the range of 40 to 60 nm. Stokes shifts of this magnitude would greatly simplify the optical filtration requirements needed to operate a scanning system and provide adequate fluorescent light signal for further processing. Unfortunately, fluorescent compounds with desirable Stokes shifts and fluorescent capabilities are not presently available.
Yet another problem is that fluorescent light intensity is attenuated in a nonlinear fashion with respect to the darkness of the underlying background. U.S. Pat. No. 4,983,817 discloses an approach to compensating for this problem that entails adding an additional detector to simultaneously measure the background reflectance light. An electronic, nonlinear algorithm is applied to the reflectance signal as a means of providing background compensation for the separately detected, fluorescent signal. This technique reportedly results in a constant amplitude signal being generated for subsequent electronic processing. Although this system is purported to work well with inks with ideal optical characteristics, i.e., having large Stokes shifts, the dyes claimed in the reference have very short lifetimes, which renders them impractical for many applications. NIR fluorophores with longer lifetimes are disclosed in U.S. Pat. Nos. 5,292,855 and 5,366,714, but these dyes do not have the large Stokes shifts needed for the operation of the scanning system described. Thus, less than ideal optical filtration constraints are imposed on the system, resulting in high loss levels of the fluorescent light. The aforementioned systems therefore do not successfully decode these longer-lived invisible marks over underlying backgrounds of widely varying densities.